1. Field of the Invention
The present invention relates to a method for preparing a hollow fiber-type separation membrane from high density polyethylene, which can improve the water permeability and molecular weight cut-off of the membranes as well as control the distribution and size of their pores, thus providing various membranes for various uses. Also, the present invention is concerned with an apparatus which is suitable for conducting the method.
2. Description of the Prior Art
There have been great advances in the development of polymer membranes through which liquid phase materials are separated. In the early stages of the development of separation membranes, the membranes for microfiltration or precision filtration were of flat structure. Recently, hollow fibers have been developed, and their application for microfiltration membranes has brought about a great improvement in penetration area. Since the development of hollow fibers, the separation membranes have found numerous applications in various fields.
For separation membranes, there have been used various materials, including polysulfones (Japanese Pat. Publication No. Sho. 56-54164), photo-crosslinked polymers (Japanese, Pat. Laid-Open Publication Nos. Sho. 52-136107 and 58-84006), fluorine-containing polymers (Japanese Pat. Laid-Open Publication No. 59-166541), acetic acid cellulose, polyamide, and polyether sulfones.
Generally, in order to provide hollow fibers with separating functions, porous structures are imparted to them by various techniques, such as solvent exchange phase inversion, phase separation, zone drawing, etc.
For example, when hollow fiber-type separation membranes are prepared by a solution spinning method using a solvent exchange phase inversion technique, a polymer is dissolved, along with a pore-forming agent, in a solvent, after which the dissolved polymer is spun in a dry spinning manner or a dry and wet spinning manner. Then, the fibers thus obtained are subject to a nonsolvent in which the solvent is exchanged with nonsolvent components to form fine pores. In the course of the diffusion of the solvent into the nonsolvent, which is a coagulating bath, finger-like structures are formed, making the hollow fiber membrane an asymmetric membrane or a symmetric membrane in which the interior is identical to the exterior.
For the purpose of improving the permeability and separative selectivity of hollow fiber membranes, the hollow fiber membranes must be formed into the thinnest possible state to minimize their penetration resistance as long as they are durable to the pressures used. A reference directed to this end can be found in Korean Pat. Publication No. 92-1258 disclosing a separation membrane in which a buffer layer is formed between an active layer and a macro pore support layer. However, since the spinning solution for the membrane comprises a polymer, a main-pore forming agent, a sub-pore forming agent, inorganic salts, and surfactant and each of these components act as a factor to affect the fiber-making procedure, it is very difficult to select a spinning composition and process condition for optimal control. In addition, expensive polymers, such as polysulfones, are used. Further, secondary waste water is caused by the four or more chemical species used, and problems in economic and environmental aspects arise in association with the process. Surfactant, which is used to improve interfacial properties of the hollow fiber-type separation membrane, is feasibly washed out by industrial water and household water, so regard must be paid to the influence according to the agents added.
Conventional materials for separation membranes suffer from a disadvantage upon disposing waste water or industrial water. For example, acetic acid cellulose or polyamide separation membranes are not suitable for high temperature processes because the membranes are poor in thermal resistance. Polysulfone separation membranes are feasibly damaged by industrial waste water because of weak resistance to chemicals. As for membranes of fluorine-containing polymers, the polymers are hard to dissolve in ordinary solvents. In addition, the fouling in which solid contents of waste water adhere to separation membranes must be solved.
Problems are also found in conventional techniques of preparing separation membranes.
In the case of a solution spinning method, first, it is not environment-affinitive because the solvent and nonsolvent used produce pollution. Further, solvents are necessary for the used polymers. Separation of the spinning solution used costs a great deal and the solution is difficult to reuse because it is composed of various components of different properties. In addition, the components, such as polymers, solvents, nonsolvents, etc., are expensive. In order to use a membrane prepared by a solution spinning method in treating drinking water, an examination must be made of the toxicity of additives, such as surfactants, to the body.
For a phase separation method, using a diluting agent and nucleating agent, a polymer which is not melted at room temperature is diluted and melted at a high temperature and cooled. Then, during the cooling of the diluted solution, there takes place crystallization or phase separation in which a polymer rich phase and a polymer lean phase are formed. The finely porous membrane thus prepared is of a honey comb structure with a symmetry. However, the membrane provides a complex penetration path for a medium, so that it shows a high penetration resistance per hour and a low penetration efficiency. In addition, the membrane is short of the basic physical properties, such as water pressure resistance, which a separation membrane must possess.
When a separation membrane is prepared by a zone drawing method, a polymer resin, which is not melted at room temperature, is first melted at a high temperature. The melt is melt-spun through an extruder to prepare an undrawn fiber. After being given desired defects by addition of a gaseous strong acid, the undrawn fiber is subjected to mechanical zone drawing in a special drawing apparatus. The hollow fiber-type separation membrane thus obtained has a surface on which fibrils are formed with micropores therebetween. Although having a disadvantage of using toxic chemicals, this zone drawing method is now regarded as the most competitive by virtue of its advantages of being low in production cost and high in production yield.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for preparing an inexpensive separation membrane superior in necessary physical properties, such as chemical resistance, in which the performance of the membrane can be changed simply by controlling spinning conditions without the aid of an additive.
It is another object of the present invention to provide a preparing method of a separation membrane which is applicable for a broad spectrum of uses and superior in permeability and separative selectivity.
It is a further object of the present invention to provide a preparing method of a separation membrane, which can improve the water permeability and molecular weight cut-off of the membranes as well as control the distribution and size of their pores.
It is still another object of the present invention to provide an apparatus for preparing a separation membrane, in which the method of the present invention can be accomplished.
In one aspect, there is provided a method for preparing a hollow fiber-type separation membrane from high density polyethylene, comprising the steps of: melt-spinning a mixture of a high density polyethylene and a diluting agent to produce a phase-separated, undrawn hollow fiber; detecting a tensile strength of the fiber during an alternative and repetitive winding and unwinding procedure between two bobbins, said fiber being drawn during the alternative and repetitive winding and unwinding procedure; reducing a rotation speed of one of the two bobbins when the detected tensile strength is larger than a predetermined value; and increasing a rotation speed of one of the two bobbins when the detected tensile strength is smaller than the predetermined value. In one embodiment of the present invention, the winding and unwinding procedure of the undrawn fiber is repeated to a number which is determined depending on a preparation condition for the undrawn fiber.
In another aspect, the present invention provides an apparatus for preparing a hollow fiber-type separation membrane of high density polyethylene, comprising: a rereeling bobbin from which an undrawn hollow fiber is unwound; a take-up bobbin around which the undrawn hollow fiber is wound; a tensile strength means which is placed between the rereeling bobbin and the take-up bobbin; a connector block through which the tensile strength means is connected to a personal computer having a multi-functional input/output board; a first servomotor and a second servomotor which are connected to the take-up bobbin and the rereeling bobbin, respectively; and a motor controller 39 whose one side is connected to the connector block with the other side being connected to the first servomotor and the second servomotor.